OCTG used in the exploration for natural gas wells and oil wells (collectively referred to below as oil wells) and production of natural gas and crude oil are steel pipes having a length of around 10 meters. In use, they are connected to each other by threaded joints until they have a length which can reach an underground oil field. In recent years, due to increases in the depth of oil wells (such as to 8,000 meters or more) and increases in the numbers of extended reachwells and horizontal wells, the number of OCTG which are connected together is continuing to increase.
Threaded joints for connecting steel pipes typically have a pin-box structure constituted by a pin having male (external) threads and a box having female (internal) threads. Types of threaded joints include integral types in which a pin is formed on one end of a steel pipe and a box is formed on the other end of the pipe and two pipes are directly connected to each other, and coupling types in which two pipes each having a pin formed on both ends thereof are connected through a coupling having a box formed on both of its ends.
A special threaded joint capable of metal-to-metal sealing is widely used as a threaded joint for OCTG which is required to exhibit good sealing performance (gas tightness) even under a high pressure and a high load. FIG. 2 is an explanatory view showing a cross section of the pin of a special threaded joint of the coupling type, i.e., the cross section of the pin formed on one end of an oil country tubular good (a steel pipe). This pin has tapered male threads 1 formed on the outer peripheral surface of its end portion and a lip 2 which is located closer to the end surface of the pin than the tapered male threads 1. The lip 2 has a tapered metal seal portion 2a formed on its outer peripheral surface to perform metal-to-metal sealing and a torque shoulder 2b formed on its end surface.
The tapered male threads of the pin are formed by rotating a steel pipe which is used as an oil country tubular good while holding the vicinity of the end of the steel pipe with a chuck, and allowing a thread cutting tool to contact the peripheral surface of the end portion of the rotated steel pipe. Tapered male threads 1 which are formed in this manner on the pin have an incomplete thread portion (a threaded portion where the thread shape is incomplete, i.e., a threaded portion where the shape of the thread is different from that of a complete thread) at the start of cutting 1a and at the end of cutting 1b. The burrs easily develops on the crest of such an is incomplete thread.
FIG. 3 schematically shows the shape of various types of burrs which can develop at the start of cutting of threads. In FIG. 3, 3 indicates a burr, 4 indicates a thread, the 2-dash chain line indicates the shape of an imaginary thread assuming that the incomplete thread formed at the start of cutting where a burr 3 developed is actually a complete thread, and the dashed line shows the direction of machining of threads. As shown in FIG. 3, burrs 3 which can develop on an incomplete thread portion at the start of thread cutting include a burr which does not lean towards the root of thread 4 and which does not project beyond the thread height of an imaginary complete thread (FIG. 3(a)), a burr which leans towards the root of thread 4 (FIG. 3(b)), and a burr which projects towards the imaginary thread and exceeds the height of the imaginary thread (FIG. 3(c)). Of these burrs, according to VAM, a specification for a special threaded joint, a burr like that shown in FIG. 3(b) which leans towards the root of thread 4 or a burr like that shown in FIG. 3(c) which projects towards an imaginary thread beyond the height of the imaginary thread is considered unacceptable.
In thread cutting of a special threaded joint, in order to suppress burrs 3 which develop in the incomplete thread portion formed at the start of thread cutting and which easily cause galling, when performing finish cutting of male threads with a thread cutting tool referred to as a chaser, as shown in FIG. 4, a portion of the threads to be cut in the beginning is cut so as to have a bevel shape (having a triangular cross section with a sloping surface). The portion of the threads which is finished to have a bevel shape (1c in FIG. 4) is referred to as a beveled portion. Also in this description, this portion will be referred to below as a beveled portion.
Even in the case where such a beveled portion is formed, as shown in FIGS. 4 and 5, burrs 3 end up developing on the beveled portion 1c and are found at the completion of machine cutting of the tapered male threads 1. This is because thread cutting of a threaded joint for OCTG is carried out using a chaser having a cutting blade capable of cutting two or three threads simultaneously. Therefore, a large cutting load is applied, and as shown in FIG. 5(a), the height of the burrs 3 is large also in the beveled portion.
Burrs 3 on the beveled portion 1c which protrude towards the height of s thread 4 as shown in FIG. 5(a) are manually removed using a deburring tool called a buff grinder. However, since such burrs have a large height, it is difficult to completely remove them using a buff grinder, and as shown in FIG. 5(b), burrs often remain partially. In addition, manual deburring is poor in working efficiency and it may involve the risk of imparting grinding scratches to the metal seal portion or the complete thread, which must not be ground.
JP 09-207027A proposes that burrs formed on a beveled portion can be eliminated using a chaser having a prescribed radius of curvature R of its edge portion. However, it is not possible to completely remove only the burrs which developed on a beveled portion during thread cutting even if the chaser proposed in that patent document is used.